Liquid crystal displays LCDs have been popular for many applications, primarily in low power areas such as battery-powered systems or small size applications. However, LCDs have suffered from several problems over the years. For example, LCDs are difficult to view in low ambient light environments and have a limited viewing angle and poor contrast.
Various examples of prior art color liquid crystal displays include U.S. Pat. Nos. 5,359,345 and 5,724,062 both issued to Hunter. The Hunter patents describe liquid crystal displays that require arranging individual pixels in rows and corresponding columns which can be expensive, complicated to manufacture, and have narrow angular view ranges with low brightness. U.S. Pat. No. 4,791,415 to Takahashi, U.S. Pat. No. 4,871,231 to Garcia, Jr.; U.S. Pat. No. 5,184,114 to Brown; U.S. Pat. No. 5,192,946 to Thompson et al.; and U.S. Pat. No. 5,317,348 to Knize also describe display systems that have similar problems.
U.S. Patents that describe panel displays using two-frequency up-conversion fluorescence include U.S. Pat. Nos. 5,684,621; 5,764,403; 5,914,807; 5,943,160; and 5,956,172 all issued to Downing. The Downing '403 patent has relevance to the present invention because it is primarily concerned with embodiments where the use of different layers for red, green and blue emitters and briefly describes some mixing of crystal type materials in a single display media. However, for the single display media, Downing '403 uses nanometer sized particles which would inherently be difficult to form, handle and disperse in a display medium.
Other known patents in these fields include U.S. Pat. No. 5,003,179 to Pollack; U.S. Pat. No. 5,051,278 to Paz-Pujalt; U.S. Pat. No. 5,245,623 to McFarlane; U.S. Pat. No. 5,622,807 to Cutler; and U.S. Pat. No. 5,846,684 to Paz-Pujalt et al. However, these patents also fail to overcome the problems with the other patents described above.
Another known up-conversion prior art reference is U.S. Pat. No. 5,089,860 issued to Deppe et al. on Feb. 18, 1992. This patent describes a quantum well device with control of spontaneous photon emission and method of manufacturing, wherein spontaneous photon emission intensity in a semiconductor quantum well is strongly influenced by a highly reflecting interface with the quantum well interface spacing being less than the optical emission wavelength of the quantum well.
Other know prior art up-conversion includes U.S. Pat. Nos. 6,327,074 and 6,501,590 issued to Bass et al. respectively on Dec. 4, 2001 and Dec. 31, 2002, which are assigned to the same assignee as the present invention. The Bass patents describe display mediums using emitting particles that are dispersed in a transparent host. The two and three dimensional color image displays include a display medium having a substantially uniform dispersion of red, green and blue visible light emitting particles sized between approximately 0.5 to approximately 50 microns therethrough. The particles can be dye doped polymethylmethacrylate (pmma) plastic, and the display medium can be pmma, acrylic plastic or glass. Other particles can be used such as rare earth doped crystals. The two dimensional display uses three laser sources each having different wavelengths that direct light beams to each of three different types of particle in the display medium. Light is absorbed by the particles which then become excited and emit visible fluorescence. Modulators, scanners and lens can be used to move and focus the laser beams to different pixels in order to form the two dimensional images having different visible colors.
U.S. Pat. No. 6,654,161 issued to Bass et al. on Nov. 25, 2003, which is also assigned to the same assignee as the present invention, describes dispersed crystallite up-conversion displays based on up conversion of near infrared light to visible light. The display medium is a transparent polymer containing particles of crystals doped with Yb.sup.3+ and other rare earth ions. The Yb.sup.3+ ions absorb light from a commercially available diode laser emitting near 975 nm and transfers that energy to the other dopant ions. Using a fluoride crystal host, NaYF.sub.4, co-doped with Tm.sup.3+ ions blue light at about 480 nm was obtained, with Ho.sup.3+ or Er.sup.3+ ions green light at about 550 nm is obtained and with Er.sup.3+ red light at about 660 nm is obtained. The display medium can be used with applications for full color, high brightness, high resolution, displays.
U.S. Pat. No. 6,844,387 issued to Bass et al. on Jan. 18, 2005, which is also assigned to the same assignee as the subject invention describes composites of inorganic luminophores stabilized in polymer hosts. The two and three dimensional display medium can have a novel transparent polymer composite containing particles of crystals doped with Yb.sup.3+ and other rare earth ions. The polymer composite creates homogeneously dispersed compositions without cracking or delamination of the film and can be used for various optical applications.
U.S. Pat. No. 6,844,387 issued to Bass et al. on Jan. 18, 2005, another patent having the same assignee as that of the subject invention discloses an optically written display. The two, three dimensional color displays can include uniform dispersion of red, green and blue visible light emitting micron particles. Pumping at approximately 976 nm can generate green and red colors having an approximately 4% limit efficiency. One light source can generate three colors with a low limit efficiency. Modulators, scanners and lens can move and focus laser beams to different pixels forming two dimensional color images. Displays can be formed from near infrared source beams that are simultaneously split and modulated with micro electro mechanical systems, spatial light modulators, liquid crystal displays, digital micro mirrors, digital light projectors, grating light valves, liquid crystal silicon devices, polysilicon LCDs, electron beam written SLMs, and electrically switchable Bragg gratings. Pixels containing Yb,Tm:YLF can emit blue light. Pixels containing Yb,Er(NYF) can emit green light, and pixels containing Yb,Er:KYF and Yb,Ef:YF.sub.3 can emit red light.
The concept of frequency up-conversion (UC) of infrared-to-visible light in rare-earth (RE) doped materials was reported more than forty years ago for the first time. The efficiency that was observed or expected for this process was low in singly doped media. It was quickly noticed that up-conversion could be made one or two orders of magnitude more efficient by using ytterbium (Yb) as a sensitizer ion in addition to the active ion: erbium (Er), holmium (Ho), or thulium (Tm).
In years past, efficient up-conversion materials were investigated, for photonic applications, such as in UC lasers (visible lasers that are pumped by infrared diode lasers), or in display applications. However, because no powerful source existed in the 980-nm region in order to excite those up-converters, no practical product came out of the research. With the development of powerful 980-nm diode lasers lead by the telecommunication industry, there can now be legitimate practical applications.
It has been noticed in the prior art that pumping conditions caused heating of the material and that higher efficiencies were obtained with low duty cycle excitation. It was also reported that for a same average input power, higher efficiencies were expected in pulsed excitation mode than in continuous wave excitation due to the quadratic nature of the process.
The effect of the pumping conditions for display applications of UC materials needs to be understood, as several technologies might be used to form the image. The infrared source can either be scanned (vector-addressed or raster-scan), or the image can be directly projected using Digital Micromirror Devices (MEMS) such as in the Texas Instrument Digital Light Processing technology. In the latter case the materials would be undergoing pulse-excitation, whereas they would be quasi-continuously excited in the second case.
U.S. Pat. No. 7,075,707 issued to Rapaport et al. on Jul. 11, 2006, and to the same assignee as that of the subject invention, describes a substrate design for optimized performance of up-conversion phosphors utilizing proper thermal management. The patent describes methods and compositions for using an up-conversion phosphor as an emitting material in a reflective display and Polymer compositions for display mediums, and red, green, blue (RGB) display mediums. Roles of the pumping duration and character on the temperature and the efficiency of the up-conversion process in (Ytterbium, Erbium or Thulium) co-doped fluoride crystals are also described. A problem with prior art up-conversion devices is limited efficiency since much of the incident pump light is back scattered by the up-converting particles and does not get used to generate visible light.